A phosphor emitting light in response to stimulation is useful in many technical fields, including fluorescent lights, electron field-emission device displays (FED), and vacuum fluorescent displays (VFD). A phosphor responsive to excitation by electrons of low energy (i.e. accelerated by a low voltage) is particularly useful, and there are particular needs for a blue-light-emitting phosphor of high spectral purity. The hue of light emission from a phosphor is often described in terms of a wavelength or range of wavelengths of emitted light, such as the wavelength of a major or dominant peak in the phosphor's emission spectrum, or by coordinates (x and y) in a CIE (Commission Internationale d'Eclairage) chromaticity diagram. Blue light is conventionally characterized in the wavelength sense by a spectrum with a dominant peak between about 380 nanometers and about 480 nanometers, e.g. around 430 nanometers (nm). Chromaticity x and y values in the region of the CIE 1931 chromaticity diagram corresponding to blue hues are in a region about x=0.15, y=0.1. Representative blue primaries in various standards for RGB display systems correspond to similar CIE 1931 chromaticity x and y values: for example, CIE spectrum primary blue x=0.167, y=0.009; NTSC standard primary blue x=0.140, y=0.080; and graphics-monitor primary blue x=0.150, y=0.070.
There is a long-standing and continuing need for phosphors emitting in the blue region of the spectrum with high spectral purity. Phosphors such as ZnO:Zn, ZnS:Au, CdWO.sub.4, and Zn.sub.2 WO.sub.5 (each having blue-green peak light emission) and ZnGa.sub.2 O.sub.4, ZnS:Zn, and ZnS:Ag (each having generally blue peak light emission) have been known in the art for many years. (See, for example, the article by H. W. Leverenz, "Phosphors Versus the Periodic System of the Elements" Proc. I.R.E. [May 1944] pp. 256-263.) U.S. Pat. No. 4,275,333 to Kagami et al. describes fluorescent compositions and low-velocity-electron excited fluorescent display devices, utilizing phosphors which include blue-light-emitting phosphors. U.S. Pat. Nos. 5,120,619 and 5,250,366 to Nakajima et al. describe rare earth tantalate and/or niobate phosphors which emit light under X-ray excitation, with peak light emission generally below 370 nm, and typically between about 320 nm and 350 nm. U.S. Pat. No. 5,138,171 to Tecotzky et al. describes a photostimulable X-ray energy absorbing halosilicate, halogermanate, or halo(silicate-germanate) phosphor having prompt light emission with a peak wavelength around 445 nm. U.S. Pat. No. 5,478,499 to Satoh et al. describes a low-velocity electron excited phosphor of blue luminous color having CIE 1931 chromaticity diagram y values of 0.05 to 0.25. U.S. Pat. No. 5,507,976 to Bringley et al. describes stabilized phosphor intermediates and storage phosphors capable of storing latent X-ray images for later release. At least some of the storage phosphors taught by the Bringley et al. patent are the products of firing combinations ("stabilized intermediates") including oxides with oxosulfur reducing agent for molecular iodine. U.S. Pat. No. 5,549,843 to Smith et al. discloses annealed alkaline earth metal fluorohalide storage phosphors including metal oxides. U.S. Pat. No. 5,571,451 to Srivastiva et al. describes a quantum-splitting oxide phosphor doped with Pr.sup.3+, which has an emission spectrum having a peak emission at 400 nm when excited by vacuum ultra-violet radiation.
Many phosphors have been developed with pigments incorporated into, attached, or coated on phosphors to modify the light emitted by the underlying phosphor in order to achieve a desired hue or a desired color temperature. For example, U.S. Pat. No. 4,152,483 describes a pigment coated phosphor and process for manufacturing it; U.S. Pat. No. 4,699,662 to Nakada et al. describes a blue pigmented phosphor; U.S. Pat. No. 5,077,088 to Jeong describes a process for preparation of a pigment-coated phosphor; and U.S. Pat. No. 5,363,012 describes a pigment-attached blue-emitting phosphor.
U.S. Pat. No. 5,455,489 to Bhargava describes displays comprising doped nanocrystal phosphors which comprise separated particles of a host compound activated by a dopant, the phosphor particles being of the order of 100 angstroms in size and exhibiting quantum-confined properties. Examples of such doped nanocrystal phosphors include ZnS:Mn.sup.2+ (yellow) and ZnS:Tb.sup.3+ (green), and II-VI host phosphors doped with Thulium (Tm), Terbium (Th), and Europium (Eu) for blue, green, and red light emission respectively.
In an article by Roger T. Williams, Steven R. Evatt, James D. Legg, and Mark H. Weichold "Blue light emission observed in a monolithic thin film edge emission vacuum microelectronic device" Journal of Vacuum Science and Technology B, V. 13, No. 2 (March/April 1995), p. 500 ff, light emission at about 488 nanometers wavelength was reported from a multi-layer phosphor structure (Al, ZnO, and W) under forward bias conditions.
Many of the phosphors known in the art are conventionally prepared by methods including grinding the phosphor composition and/or its precursors to a powder having a particle size distribution suitable for a particular purpose. For example, phosphors prepared for X-ray storage panels may have a median particle size of about 0.5 to 40 micrometers. U.S. Pat. No. 5,536,383 to Tran Van et al. teaches the use of a non-aqueous suspension for the deposition of luminescent materials, particularly phosphors, by electrophoresis. Phosphor powder particles in the suspension have the finest possible grain size, e.g. approximately 1 to 10 micrometers. In a field-emission-excited cathodoluminescence display structure taught by Tran Van et al. the phosphor is deposited by electrophoresis onto a transparent, conductive coating, e.g. of indium and tin oxide (ITO), on a transparent insulating substrate. U.S. Pat. No. 5,601,751 to Watkins et al. discloses a manufacturing process for high-purity phosphors of small average particle size, exhibiting sufficient luminescent efficiency for utility in field emission displays. In the process of Watkins et al., a precursor mixture including a host lattice material and a dopant starting material is milled to obtain a sized precursor mixture having an average precursor particle size less than about 1 micrometer. Particle size growth during subsequent heating for infiltration of the dopant into the host lattice structure is held to a minimum, e.g. less than about 100% or preferably less than about 50%.
Phosphors intended to be used in vacuum fluorescent displays or display devices of the cold-cathode field-emission type should not contain substances that can contaminate the cathode, causing deterioration of electron emission. Thus phosphors containing sulfur (S), cadmium (Cd), or cesium (Cs), for example have not found favor for such applications, as those elements can cause contamination in the displays. U.S. Pat. No. 5,619,098 to Toki et al. discloses a phosphor free of S and Cd, made from compounds of titanium (Ti), alkaline earth metal, and an element of group 13 of the periodic table.